Conventional portable printers use a roll of wound stock material, such as paper or label stock, which is loaded into the printer such that the paper from the roll will properly feed and align with a thermal print head for printing. These rolls may be in different widths such that labels of different widths may be printed.
A roll may be side-loaded and centered onto a spindle as shown in U.S. Pat. No. 5,860,753, or top-loaded and centered, as in the label printer manufactured by Zebra Technologies, Corp., Camarillo, Calif., model no. P2242. Printers providing for a top-loaded roll have a cavity to receive the roll and two rotatable spindle members are urged by spring or springs into the tubular core of the rolls into a centered position with respect to the print head of the printer. One problem with the top-loaded portable printer is that when the printer is dropped or otherwise receives an accidental impact, the roll can disengage from the spindle members, negatively impacting printer function or require the operator to reset the roll between the spindle members.
Regardless of the loading approach used, the print head of a typical portable printer is of a length sufficient to print the widest paper for that printer so as to accommodate the range of roll widths. When rolls are of a width less than the print head length, the print head's width exceeds the paper width. Typically, the user of the portable printer must assure that the roll is of a proper width for the information to be printed, otherwise the printing may extend beyond one or both sides of the paper from the roll, or from one side of the roll from a non-centered roll. Examples of portable printers with non-centered rolls are shown for example in U.S. Pat. Nos. 5,267,800 and 5,447,379. Thus, printing elements of the print head may be utilized corresponding to areas outside the width of the roll, which over time will likely damage the print head. This damage is due to heat buildup by printing elements that are not in contact with the paper, and therefore, not able to transfer heat to the paper. Thus, it is desirable to automatically align printing by a portable printer with the width of the roll.
In larger ink jet printers a reflective sensor may be provided under the carriage for detecting the width of sheets of paper transported from a stack of paper. Such ink jet printers, are described, for example, in U.S. Pat. Nos. 5,398,049, and 6,007,184. A paper width detector LED and paper width sensor are described in the ink jet printer of U.S. Pat. No. 6,193,344. However, such ink jet printers due to their weight or size cannot be practically worn or hand carried and are not part of any centering mechanism for a roll.